Upgrade ke Jekyll 4.0.0

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Jekyll 4.0 comes with some major changes, notably:
* Our `link` tag now comes with the `relative_url` filter incorporated into it.
You should no longer prepend `{{ site.baseurl }}` to `{% link foo.md %}`
For further details: https://github.com/jekyll/jekyll/pull/6727
* Our `post_url` tag now comes with the `relative_url` filter incorporated into it.
You shouldn't prepend `{{ site.baseurl }}` to `{% post_url 2019-03-27-hello %}`
For further details: https://github.com/jekyll/jekyll/pull/7589
* Support for deprecated configuration options has been removed. We will no longer
output a warning and gracefully assign their values to the newer counterparts
internally.
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